A light source device using an optical semiconductor element such as an LED and a wavelength conversion layer such as a fluorescent layer in combination has been in widespread use. As a result of recent trends toward higher brightness, such a light source device has been applied in a wider range of applications including general lighting and in automobile and other vehicle headlights. This type of light source device is considered to be able to provide a wider range of applications if it continues to increase in terms of its brightness.
The brightness of the aforementioned light source device using an optical semiconductor element and a wavelength conversion layer in combination may be increased by applying a large current to the optical semiconductor element to increase the intensity of excitation light emitted from the optical semiconductor element. However, the actual situation is such that heat is generated in the wavelength conversion layer to cause discoloration of a resin component or temperature quenching of a wavelength conversion material in the wavelength conversion layer, leading to reduction of the intensity of fluorescent light. This may lead to saturation or reduction of the intensity of emitted light, making it difficult to increase the brightness of a light source device utilizing an optical semiconductor element and a wavelength conversion layer in combination.
In order to form a wavelength conversion layer into a fixed shape with a high degree of reproducibility, the wavelength conversion layer is generally formed by blending wavelength conversion material powder and a resin component to prepare paste thereof, and applying the paste by a printing method and the like. The aforementioned discoloration of the resin component in the wavelength conversion layer is a phenomenon where the resin component is discolored if the resin component is heated to a temperature of about 200° C. or higher. A resin component is originally transparent. Accordingly, if discolored with heat, the resin component absorbs part of the excitation light from an optical semiconductor element or fluorescent light from a wavelength conversion layer, placing an obstacle to increasing the brightness.
The aforementioned temperature quenching of a wavelength conversion material is a phenomenon where the intensity of fluorescent light is reduced if the wavelength conversion material is heated. As a result of reduction of the intensity of fluorescent light due to temperature quenching, energy not having been converted to fluorescent light becomes heat to increase the amount of heat generation of the wavelength conversion material. This increases the temperature of the wavelength conversion material further to promote the temperature quenching, leading to further reduction of the intensity of fluorescent light. Accordingly, the temperature quenching of a wavelength conversion material due to heat generation also places an obstacle to increasing the brightness in the device or lamp.
The aforementioned characteristics and problems may be solved or at least addressed by a light source suggested by Japanese Patent Application Laid-Open No. 2006-005367. This light source uses a wavelength conversion layer not containing resin. In this case, the wavelength conversion layer does not contain a resin component, so that discoloration of a resin component does not occur. Further, the wavelength conversion layer can be a ceramic layer made of a wavelength conversion material of low sensitivity to temperature. This avoids generation of temperature quenching, making it possible to increase brightness. The light source of Japanese Patent Application Laid-Open No. 2006-005367 is intended to let heat generated in a fluorescent layer 92 dissipate to an optical semiconductor element (fixed light source) 95 by connecting the fluorescent layer 92 directly to the optical semiconductor element (fixed light source) 95 as shown in FIG. 1.
The conventional light source device shown in FIG. 1 where the optical semiconductor element (fixed light source) 95 and the fluorescent layer 92 are directly connected makes use of fluorescent light and excitation light. The fluorescent light to be used can be part of the light (fluorescent light) emitted from the fluorescent layer 92 excited with excitation light from the optical semiconductor element (fixed light source) 95, and which is to exit in a direction opposite to the optical semiconductor element (fixed light source) 95. The excitation light to be used is light emitted from the optical semiconductor element (fixed light source) 95, and which is transmitted through the fluorescent layer 92 without being absorbed in the fluorescent layer 92. To be specific, the light source device of FIG. 1 adopts a transmission system making use of light transmitted through the fluorescent layer 92.
Light emitted from the fluorescent layer 92 can include the aforementioned transmitted light and additionally, reflecting light that is specifically light going back to the optical semiconductor element (fixed light source) 95 after being reflected off an interface between the optical semiconductor element 95 and the fluorescent layer 92. This light (reflecting light) can be absorbed in the optical semiconductor element (fixed light source) 95 again, so that it cannot be used as illuminating light.
The light source device of FIG. 1 is intended to let heat of the fluorescent layer 92 be dissipated to the optical semiconductor element (fixed light source) 95. Meanwhile, if the intensity of excitation light from the optical semiconductor element (fixed light source) 95 is increased, heat can be generated not only in the fluorescent layer 92 but also in the optical semiconductor element (fixed light source) 95. This means that heat generated in the fluorescent layer 92 is caused to be dissipated to the optical semiconductor element (fixed light source) 95 that also generates heat, leading to poor efficiency of heat dissipation.
Accordingly, adoption of the transmission system and the poor efficiency of dissipation of heat generated in the fluorescent layer 92 of the light source device of FIG. 1 place limitations on the increase of brightness.